I can understand your concern for your health. Since you have a poor appetite with a weak digestive system, I would suggest you to eat small meals at frequent intervals to ensure you eat more and the food gets digested easily. Heavy meals must be avoided when your digestion is poor. Also poor appetite does not permit adequate food intake if you follow a 3 meal pattern throughout the day. So,in your case you need 3 small meals with 3-4 in between snacks. Due to deficiency of Vitamin B12, you have developed mouth ulcers which might have made eating more difficult. You need to eat energy dense nutrient dense foods like eggs in boiled form, you can eat mutton, chicken or fish also if you are a non-vegetarian as these foods take care of many nutrient deficiencies (vitamin B12, folic acid, iron) which result in mouth ulcers. Have simple easily digestible foods like rice /pulses/khichdi/idli (indian diet) alongwith curd/yogurt/milk in the form of milkshakes using full cream milk. Eat whenever you are hungry. Keep energy dense snacks with you when you go out like sandwiches, peanut and jaggery bars. Due to poor appetite, you are kindly advised not to do the following: 1. Having coffee/tea more than 2 cups in a day and just before/after meals. 2. Avoid filling your plate with too much food at a time or too heavy food like rich gravies, fried foods etc. The very sight of a plate full of food reduces appetite. 3. Drinking fluids often and especially before meals which will reduce food intake. 4. Do not eat in a hurry. Instead eat in a relaxed atmosphere conducive to good appetite. Hope you will benefit from my advice. Take care